I?m not sure what part of that is hard to understand. I copy and pasted a good ovutline or what 1, 2, 3 and 4 satellites can do for your location. I even posted the damn pictures.

It took rocket science to get them into geosynchronous orbits, but it doesn?t take rocket science to understand what is being discussed and how a technical websites about GPS trumps your consumer user manuals.

As a quick clarification the GPS constellation is not at geosynchronous orbit

I don?t think they are wrong, either, I even posted how less than 3 can determine your position, but three is to determine your position on a plane which satellites 12,000 miles up measuring a tiny object on the surface of a sphere can?t accurately accomplish without 4 satellites. Since they are geosynchronous and the Earth?s surface isn?t changing with any great degree (even Mount Everest is only 5.5 miles above sea level) they will often do a decent job with 3 with some assumed data on elevation.

But, it's the other way around. You can position accurately on the surface of the earth with three satellites, but you need four in a three dimensional space which is the case with flight, where height is a critical component.

Even if I'm traveling in the mountains, I rarely care about how high I am, but I care WHERE I am in relation to other locations. In that case, three will work. Four MAY be required for tracking purposes, but I'm not certain of that, as my older models didn't need more than three for routing and waypoints.

The way they do all of this is very clever, and looking at the math in wikipedia's article makes it pretty clear to me as to how clever it all is.

The manual is saying exactly what you're saying it isn't saying. How can you disagree with that? If you're saying the manual is wrong, that's different. But it very definitely is saying that three satellites are needed for horizontal positioning. It's quite clear.

I think this disagreement is about the difference between theory and practice. Theoretically, it requires 4 satellites to determine an unambiguous position. Practically, a location on the "surface" can be determined with 3 by a GPS unit assuming you aren't out in "space" -- you are in one of two places, and one is discarded as not within expected parameters.

As a quick clarification the GPS constellation is not at geosynchronous orbit

That's true, which is why emphemeris data needs to be constantly updated. Signal timing corrections requiring approx 30 seconds to download the entire packet. But that could take your device longer than 30 seconds to receive it if you "discovered" it midway thru the data send. It repeats every (I think) 30 seconds, 24 hours a day.

I?m not sure what part of that is hard to understand. I copy and pasted a good ovutline or what 1, 2, 3 and 4 satellites can do for your location. I even posted the damn pictures.

It took rocket science to get them into geosynchronous orbits, but it doesn?t take rocket science to understand what is being discussed and how a technical websites about GPS trumps your consumer user manuals.

Just a technicality, but note that the GPS constellation is not in geosychronous orbit. Not only would that require them all to be over the equator, but would also place them so far out that signal-to-noise issues at the receivers would be difficult to handle.

I think this disagreement is about the difference between theory and practice. Theoretically, it requires 4 satellites to determine an unambiguous position. Practically, a location on the "surface" can be determined with 3 by a GPS unit assuming you aren't out in "space" -- you are in one of two places, and one is discarded as not within expected parameters.

There is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate.

All the disagreement and doubting won't change that from fact. I already gave you a simple test. Did anyone bother to try it, or would you rather just continue guessing that I must be wrong.

This is the problem when reading an article from Wikipedia. They usually get it pretty correct, not not completely correct. This is one of those times. Their error is the usuall Wikipedia issue of omission. In order to understand this issue, you really need to read the number 59 footnote, which take you to the original article from which the information about P codes and the rest came from.

If you do, you will see a number of I treating statements. One is that "mostly millitary" equipment uses these codes. Other equipment dies as well. Then there is the date, which is 1994. These codes were open up in 2000, as has been mentioned earlier. The military may encrypt the information going to and from their own equipment, but it's still there, in unencrypted form for everyone else today.

There is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate.

All the disagreement and doubting won't change that from fact. I already gave you a simple test. Did anyone bother to try it, or would you rather just continue guessing that I must be wrong.

Well, while we're providing links to our arguments, were still waiting for yours. I'm conceding that you may very well be correct about everything you've said, despite information showing differently. But until you provide a good link, we don't know. I would be very happy to acknowledge whatever you say, if you can show it to us in a way that invalidates whatever else we've got. It's up to you.

As a quick clarification the GPS constellation is not at geosynchronous orbit

Quote:

Originally Posted by muppetry

Just a technicality, but note that the GPS constellation is not in geosychronous orbit. Not only would that require them all to be over the equator, but would also place them so far out that signal-to-noise issues at the receivers would be difficult to handle.

I wrote geosynchronous. Geostationary specifically refers to a specific kind of geosynchronous orbit at 0° latitude (equator). Geosynchronous orbits can be elliptical (as well as other types of orbits I can’t remember).

I specifically choose not to refer to them as "elliptical geosynchronous orbits” since we can’t even agree on how many satellites it takes to determine a point on a three dimensional Cartesian coordinate system.

Unless I’m missing something and you two are saying these satellites don’t continuously follow the same elliptical path.

There is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate.

How many satellites are required by my Garmin device to determine latitude and longitude? They say three, I have observed this in practice. In my 'layman' understanding of this is that the elevation is assumed to be the last known elevation or sea level if there is no fourth satellite.

I imagine that you have a sphere with a line intersecting it through the center of the sphere. There are two locations where the line intersects the surface of the sphere. One location is very near where you were last known to be and the other is on the opposite side to the earth. The one on the opposite side is not relevant. Assuming that the software in the Garmin is smart enough to work with three satellites and the assumption that you are on the earth's surface, only three satellites are necessary. I have never received a message on the device that there aren't enough satellites to calculate my position when I have at least three.

There is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate. ...

Quote:

Distances from three satellites usually intersect at two points, and if you're not flying around, one of these points will be on Earth's surface.

So, practically, a GPS, using appropriate heuristics, and assuming the earth is a sphere can determine your position on the earth's surface with 3 satellites. You may or may not have some knowledge of GPS theory, but, as usual, you don't know what you are talking about.

Well, while we're providing links to our arguments, were still waiting for yours. I'm conceding that you may very well be correct about everything you've said, despite information showing differently. But until you provide a good link, we don't know. I would be very happy to acknowledge whatever you say, if you can show it to us in a way that invalidates whatever else we've got. It's up to you.

How many more links would you like? I've posted at least two professional and gps-industry specific sources as well as listed my own qualifications. I explained the source of your misunderstanding of the Garmin manual. And I've given you a test to prove me wrong.

Ok that site is really bad. Please just find the words: "GPS requires four satellites" on an authoritative source and link to it like I did with the documents posted. I'm not going to read through a whole website trying find what you are claiming is in there somewhere. And by the way the pictures on the page you referenced show only 3 satellites.

Correct. But still a reason that it does need to be updated regularly. A non-geosynchronous orbit would be an example of an orbital variation.

Well that's not quite correct. A geosynchronous orbit is one that matches the rotation of the earth, but in all other respects it is just an orbit like any other, i.e. from the point of view of the satellite the fact that the earth has the same rotation period has no effect on the mechanics of the orbit. All orbits, geosynchronous or not, are described by ephemeris parameters, which only need to be updated due to orbital shifts - either intentional orbit corrections or natural variations due to deviation of the system from the ideal model used to construct the data.

If you're not willing to take 3 minutes to get as far as Step 3, I've wasted hours to try and explain. I 'm not going to bother wasting another hour on finding and posting links that you can't be bothered to read.

It's apparently only important to you as something to argue about rather than take the time to understand. And that's fine. Not everyone is curious about how GPS works, only that it does.

Please just find the words: "GPS requires four satellites"

Now you’re just being obtuse.

Seriously, what part of post #101 is confusing about how 1, 2, 3 and 4 satellites work? What part of Cartesian coordinate system are you not getting? There is only one way to get a single point in a 3-dimensional space. As it’s been stated ad nauseam it assumes your terrestrial position but it actually doesn’t know it until 4th satellite is used. Don’t they teach geometry in middle school any more? I’m done with this conversation. If I wanted to teach elementary maths I’d get a job at a school.

Well that's not quite correct. A geosynchronous orbit is one that matches the rotation of the earth, but in all other respects it is just an orbit like any other, i.e. from the point of view of the satellite the fact that the earth has the same rotation period has no effect on the mechanics of the orbit. All orbits, geosynchronous or not, are described by ephemeris parameters, which only need to be updated due to orbital shifts - either intentional orbit corrections or natural variations due to deviation of the system from the ideal model used to construct the data.

Ah, finally someone besides Solipsism that has taken the time to try and understand the whys and whatnots of GPS. Welcome to the discussion Muppetry

## Comments

326membersolipsismI?m not sure what part of that is hard to understand. I copy and pasted a good ovutline or what 1, 2, 3 and 4 satellites can do for your location. I even posted the damn pictures.

It took rocket science to get them into geosynchronous orbits, but it doesn?t take rocket science to understand what is being discussed and how a technical websites about GPS trumps your consumer user manuals.

As a quick clarification the GPS constellation is not at geosynchronous orbit

25,726membermelgrossIt's not. You need three satellites.

You?re literally thinking two dimensionally and ignoring what 1, 2 and 4 satellites are establishing.

33,504membersolipsismI don?t think they are wrong, either, I even posted how

less than3 can determine your position, but three is to determine your position on a plane which satellites 12,000 miles up measuring a tiny object on the surface of a sphere can?t accurately accomplish without 4 satellites. Since they are geosynchronous and the Earth?s surface isn?t changing with any great degree (even Mount Everest is only 5.5 miles above sea level) they will often do a decent job with 3 with some assumed data on elevation.But, it's the other way around. You can position accurately on the surface of the earth with three satellites, but you need four in a three dimensional space which is the case with flight, where height is a critical component.

Even if I'm traveling in the mountains, I rarely care about how high I am, but I care WHERE I am in relation to other locations. In that case, three will work. Four MAY be required for tracking purposes, but I'm not certain of that, as my older models didn't need more than three for routing and waypoints.

The way they do all of this is very clever, and looking at the math in wikipedia's article makes it pretty clear to me as to how clever it all is.

6,846membermelgrossThe manual is saying exactly what you're saying it isn't saying. How can you disagree with that? If you're saying the manual is wrong, that's different. But it very definitely is saying that three satellites are needed for horizontal positioning. It's quite clear.

I think this disagreement is about the difference between theory and practice. Theoretically, it requires 4 satellites to determine an unambiguous position. Practically, a location on the "surface" can be determined with 3 by a GPS unit assuming you aren't out in "space" -- you are in one of two places, and one is discarded as not within expected parameters.

23,947memberjohnnyb0731As a quick clarification the GPS constellation is not at geosynchronous orbit

That's true, which is why emphemeris data needs to be constantly updated. Signal timing corrections requiring approx 30 seconds to download the entire packet. But that could take your device longer than 30 seconds to receive it if you "discovered" it midway thru the data send. It repeats every (I think) 30 seconds, 24 hours a day.

3,331membersolipsismI?m not sure what part of that is hard to understand. I copy and pasted a good ovutline or what 1, 2, 3 and 4 satellites can do for your location. I even posted the damn pictures.

It took rocket science to get them into geosynchronous orbits, but it doesn?t take rocket science to understand what is being discussed and how a technical websites about GPS trumps your consumer user manuals.

Just a technicality, but note that the GPS constellation is not in geosychronous orbit. Not only would that require them all to be over the equator, but would also place them so far out that signal-to-noise issues at the receivers would be difficult to handle.

3,331memberGatorguyThat's true, which is why emphemeris data needs to be constantly updated.

The ephemeris data only require updating due to orbital variations - not specifically because the orbits are not geosynchronous.

23,947memberareason that it does need to be updated regularly. A non-geosynchronous orbit would be an example of an orbital variation.23,947memberanonymouseI think this disagreement is about the difference between theory and practice. Theoretically, it requires 4 satellites to determine an unambiguous position. Practically, a location on the "surface" can be determined with 3 by a GPS unit assuming you aren't out in "space" -- you are in one of two places, and one is discarded as not within expected parameters.

There is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate.

All the disagreement and doubting won't change that from fact. I already gave you a simple test. Did anyone bother to try it, or would you rather just continue

guessingthat I must be wrong.33,504membernoirdesirThe P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key.

This is the problem when reading an article from Wikipedia. They usually get it pretty correct, not not completely correct. This is one of those times. Their error is the usuall Wikipedia issue of omission. In order to understand this issue, you really need to read the number 59 footnote, which take you to the original article from which the information about P codes and the rest came from.

If you do, you will see a number of I treating statements. One is that "mostly millitary" equipment uses these codes. Other equipment dies as well. Then there is the date, which is 1994. These codes were open up in 2000, as has been mentioned earlier. The military may encrypt the information going to and from their own equipment, but it's still there, in unencrypted form for everyone else today.

http://www.kowoma.de/en/gps/signals.htm

33,504memberGatorguyThere is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate.

All the disagreement and doubting won't change that from fact. I already gave you a simple test. Did anyone bother to try it, or would you rather just continue

guessingthat I must be wrong.Well, while we're providing links to our arguments, were still waiting for yours. I'm conceding that you may very well be correct about everything you've said, despite information showing differently. But until you provide a good link, we don't know. I would be very happy to acknowledge whatever you say, if you can show it to us in a way that invalidates whatever else we've got. It's up to you.

25,726memberjohnnyb0731As a quick clarification the GPS constellation is not at geosynchronous orbit

muppetryJust a technicality, but note that the GPS constellation is not in geosychronous orbit. Not only would that require them all to be over the equator, but would also place them so far out that signal-to-noise issues at the receivers would be difficult to handle.

I wrote geosynchronous. Geostationary specifically refers to a specific kind of geosynchronous orbit at 0° latitude (equator). Geosynchronous orbits can be elliptical (as well as other types of orbits I can’t remember).

I specifically choose not to refer to them as "elliptical geosynchronous orbits” since we can’t even agree on how many satellites it takes to determine a point on a three dimensional Cartesian coordinate system.

Unless I’m missing something and you two are saying these satellites don’t continuously follow the same elliptical path.

11,510memberGatorguyThere is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate.

How many satellites are required by my Garmin device to determine latitude and longitude? They say three, I have observed this in practice. In my 'layman' understanding of this is that the elevation is assumed to be the last known elevation or sea level if there is no fourth satellite.

I imagine that you have a sphere with a line intersecting it through the center of the sphere. There are two locations where the line intersects the surface of the sphere. One location is very near where you were last known to be and the other is on the opposite side to the earth. The one on the opposite side is not relevant. Assuming that the software in the Garmin is smart enough to work with three satellites and the assumption that you are on the earth's surface, only three satellites are necessary. I have never received a message on the device that there aren't enough satellites to calculate my position when I have at least three.

6,846memberGatorguyThere is no disagreement between the theory and in practice. In theory 4 satellites are needed to determine an accurate position estimate. In practice 4 satellites are needed to determine an accurate position estimate. ...

Distances from three satellites usually intersect at two points, and if you're not flying around, one of these points will be on Earth's surface.

So, practically, a GPS, using appropriate heuristics, and assuming the earth is a sphere can determine your position on the earth's surface with 3 satellites. You may or may not have some knowledge of GPS theory, but, as usual, you don't know what you are talking about.

23,947membermelgrossWell, while we're providing links to our arguments, were still waiting for yours. I'm conceding that you may very well be correct about everything you've said, despite information showing differently. But until you provide a good link, we don't know. I would be very happy to acknowledge whatever you say, if you can show it to us in a way that invalidates whatever else we've got. It's up to you.

How many more links would you like? I've posted at least two professional and gps-industry specific sources as well as listed my own qualifications. I explained the source of your misunderstanding of the Garmin manual. And I've given you a test to prove me wrong.

I'll try one more time. This one has pictures

http://www.trimble.com/gps/howgps.shtml

NOTE: Be sure to read the whole thing, not just the first page. Step 3 and it's explanation of timing is important.

11,510memberGatorguyI'll try one more time. This one has pictures

http://www.trimble.com/gps/howgps.shtml

Ok that site is really bad. Please just find the words: "GPS requires four satellites" on an authoritative source and link to it like I did with the documents posted. I'm not going to read through a whole website trying find what you are claiming is in there somewhere. And by the way the pictures on the page you referenced show only 3 satellites.

3,331memberGatorguyCorrect. But still

areason that it does need to be updated regularly. A non-geosynchronous orbit would be an example of an orbital variation.Well that's not quite correct. A geosynchronous orbit is one that matches the rotation of the earth, but in all other respects it is just an orbit like any other, i.e. from the point of view of the satellite the fact that the earth has the same rotation period has no effect on the mechanics of the orbit. All orbits, geosynchronous or not, are described by ephemeris parameters, which only need to be updated due to orbital shifts - either intentional orbit corrections or natural variations due to deviation of the system from the ideal model used to construct the data.

23,947memberIt's apparently only important to you as something to argue about rather than take the time to understand. And that's fine. Not everyone is curious about how GPS works, only that it does.

25,726membermstonePlease just find the words: "GPS requires four satellites"

Now you’re just being obtuse.

Seriously, what part of post #101 is confusing about how 1, 2, 3 and 4 satellites work? What part of Cartesian coordinate system are you not getting? There is only one way to get a single point in a 3-dimensional space. As it’s been stated

ad nauseamit assumes your terrestrial position but it actually doesn’t know it until 4th satellite is used. Don’t they teach geometry in middle school any more? I’m done with this conversation. If I wanted to teach elementary maths I’d get a job at a school.23,947membermuppetryWell that's not quite correct. A geosynchronous orbit is one that matches the rotation of the earth, but in all other respects it is just an orbit like any other, i.e. from the point of view of the satellite the fact that the earth has the same rotation period has no effect on the mechanics of the orbit. All orbits, geosynchronous or not, are described by ephemeris parameters, which only need to be updated due to orbital shifts - either intentional orbit corrections or natural variations due to deviation of the system from the ideal model used to construct the data.

Ah, finally someone besides Solipsism that has taken the time to try and understand the whys and whatnots of GPS. Welcome to the discussion Muppetry